For the printing of coloured originals, particularly photographic negatives, it is known to carry out the exposure operation in the form of three successive part-exposures, each in one additive primary colour, and at the same time control the action of the printing light on the printing material separately in accordance with the individual primary colours. A white printing light source is usually used for this purpose, colour filters in the individual primary colours being successively swung into the path of the rays of said light source. The disadvantage of this method, in which the part-exposures are carried out successively, is that the total exposure time is relatively long as a result.
It is also known first to expose the printing material with white light and then successively interpose colour filters in the additive primary colours in the path of the light, the periods of interposition in each case being sufficient for the required amounts of printing light in the primary colours to reach the printing material during the exposure without the filter and during the successive subsequent exposures, in the primary colours. This known method, however, gives only a relatively small reduction of the printing time.
Instead of using a single white printing-light source and colour filters, it is also known to use three printing lamps of adjustable intensity which each emit light in one of the three additive primary colours, and to expose the printing material to the three primary colours simultaneously. Although this method does give a reduction of the total exposure time, the printing light produced by each lamp is low in relation to the total lamp power required, and the method is therefore uneconomic. Of the total light emitted by each printing lamp, the only fraction effectively used is the fraction containing the particular primary colour for the printing operation. The remaining light fractions are for the most part converted into heat as a power loss.
It is an object of the invention to provide an improved light source for providing light of adjustable colour, primarily for use in printing coloured originals. However, the source is capable of application in other fields where a light source that is easily adjustable as to colour is required.